Slave bus subscriber for a serial data bus

ABSTRACT

The invention relates to a slave bus subscriber for a serial data bus with a master bus subscriber, wherein the slave subscriber recognizes the bit rate of a data packet received over the data bus, whose header has a sync break field, a sync field and an ID field, with the help of the header of the data packet in such a manner that the periods between falling edges of bits having known bit intervals at least of the sync field and of the sync break field are evaluated and the bit rate is determined from the evaluated periods.

The invention relates to a slave bus subscriber for a serial data bus towhich additionally at least one master bus subscriber as well aspossibly further slave bus subscribers are connected.

Such serial data bus systems are put to multiple use in vehicles fortransmitting control signals and data to peripheral units. Such a knownserial data bus system is, for example, the LIN data bus that is usedfor purposes for which no extremely strict requirement as regardstransmission speed or reliability is made.

In such a serial data bus system, mostly one master bus subscriber andpossibly several slave bus subscribers are provided. Particularly theslave bus subscribers are to be realized in a cost-effective manner, sothat they do not generally have any quartz. However, to still enableserial communication at the required bit rate or Baud rate with such aslave bus subscriber, a header having a sync field, i.e. a field with,for example, 10 bits (including start and stop bits) and having knownbit values is generally provided in the data bus protocols of suchserial data buses, so that a bit rate or Baud rate can be determinedfrom the time sequence of the edges of the individual bits. But sincesuch sync fields are generally relatively short, frequently having 10bits in particular (including start and stop bits), the determination ofthe bit rate is not very accurate. Furthermore, in many applicationsinterference is superimposed on the serially transmitted signals andthus also on the sync field. In addition, the signals have jitter i.e.time based errors, so that the predefined signal sequence allows only animprecise determination of the bit rate on account of its limitedlength.

All said problems may cause a quite inaccurate determination of the bitrate in practice.

It is an object of the present invention to indicate a slave bussubscriber which affords a more exact determination of the bit or Baudrate, as the case may be.

This object is achieved for a first embodiment of the invention by thefeatures of the patent claim 1:

A slave bus subscriber for a serial data bus with a master bussubscriber, wherein the slave bus subscriber recognizes with the help ofthe header of the data packet the bit rate of a data packet whose headerhas a sync break field, a sync field and an ID field and is receivedover the data bus, so that the periods between edges of bits havingknown bit intervals of at least the sync field and the sync break fieldare evaluated and the bit rate is determined from the evaluated periods.

For recognizing the bit rate, the slave bus subscriber according to theinvention evaluates not only the bits of the sync field actuallyprovided for it, but also known bits of an additionally transmitted syncbreak field as well as preferably also the known bits of an ID fieldfurther transmitted in the header.

A requirement for the evaluation of these further fields is that theycontain bits having known position and known values, whose positions andthus mutual intervals of the bits must be known. Thus with the knownpositions and known bit values the time sequence of the edges of theseknown bits can also be evaluated. This takes place, in principle, in thesame manner as the evaluation of the bits of the sync field. Distinctlygreater accuracy can be achieved, however, by using other bits of otherfields, for example, the accuracy of the bit rate detection is tripledby using a total of 24 bits of the first two fields instead of the 8bits customarily used. The bit rate can be determined in a manner knownper se from the established intervals of the evaluated edges of theknown bits of these two fields up to the beginning of the third field(Note: the falling edge of the start bit of the third field is needed!).

Furthermore, a clearly reduced susceptibility to interference and jitteris achieved by the broader base i.e. the higher number of bits on whichthe bit rate recognition is based.

Furthermore, there is the advantage that a slave bus subscriber workingin accordance with the invention can be used in a data bus in commonwith slave bus subscribers that do not evaluate several fields for thebit rate recognition, but that use only 1 field for the bit raterecognition, as provided in the state of the art.

Advantageously, the start and stop bits of the fields can be used fordetermining the intervals of certain bits, as is provided according toone embodiment of the invention as claimed in claim 2, because thesestart and stop bits have known positions and values.

The evaluation of known bits i.e. bits of known values and knownpositions within the three fields assumes that also the stop bit of thesync break field has a specified time length, preferably the periodpredefined for 1 bit or a predefined number of stop bits. This is oftenthe case in known systems. However, there may also be master bussubscribers in systems, which master bus subscribers do not adhere tothese rules and select the periods or number of stop bits of the syncbreak field differently or variably. Therefore, it is proposed accordingto another embodiment of the invention as claimed in claim 3 that thistime length is signaled to the slave or programmed in the slave if thetime length of the stop bit of the sync break field is known.

Another embodiment of the invention as claimed in claim 4 describesspecial bits, which are generally fixedly predefined in theabove-described fields of the header and whose values are known, whichcan thus be used advantageously for the bit rate recognition asinvented.

Another embodiment of the invention as claimed in claim 5 provides thatseveral bits of the ID field, where known, are incorporated in the bitrate recognition. If the ID code of the master bus subscriber is known,which is generally the case, a further improvement in the accuracy andsecurity of bit rate recognition is provided.

As already mentioned in the opening paragraph, the slave bus subscriberas invented can be advantageously used in a LIN data bus, which has theabove-mentioned three fields of the header according to itsstandardization, wherein certain bits are defined, on which the bit raterecognition can be based even for unknown contents, especially of the IDfield.

These and other aspects of the invention are apparent from and will beelucidated, by way of non-limitative example, with reference to theembodiments described hereinafter.

In the drawings,

FIG. 1: shows a block circuit diagram of the slave bus subscriber asinvented and

FIG. 2: shows a time diagram of a header of a data packet.

FIG. 1 shows a slave bus subscriber 1 as invented in the form of a blockcircuit diagram, which slave bus subscriber is connected to other bussubscribers over a serial data bus DB. These bus subscribers, which arenot pointed out in any further detail in FIG. 1, are particularly masterbus subscribers and possibly other slave bus subscribers.

The slave bus subscriber shown in FIG. 1 has a transceiver 2 thatestablishes the physical connection to the data bus DB, which is aserial data bus.

The data bus may be, for example, a what is called LIN data bus, whichis often used in vehicles for transmitting control data not relevant tosafety.

An oscillator 3 is provided in the slave bus subscriber 1, whichoscillator has a low frequency accuracy, thus need not be a quartzoscillator. The construction of the oscillator provides that thestructure of the slave bus subscriber can be comparativelycost-effective.

FIG. 1 further shows in the slave bus subscriber 1 a circuit block 4 forbit rate or Baud rate recognition. The circuit block 4 for the bit andBaud rate recognition receives for this purpose, on the one hand, theoscillator signal of the oscillator 3 and, on the other hand, the datathat were received serially over the data bus DB and passed on to thecircuit block 4 by means of the transceiver 2 for bit rate recognition.

These data further reach a serial-to-parallel converter 5, whichconverts the serially received data into parallel data. These paralleldata are passed on to a control 7 over a parallel data bus 6 provided inthe slave bus subscriber 1, which control 7 carries out an evaluation ofthe data, which, however, in a way not further shown in more detail inFIG. 1, also controls the other circuit blocks of the slave bussubscriber 1.

In order to execute the oscillator 3 shown in FIG. 1 as simple aspossible, thus for example without the need to provide any quartzoscillator and also to execute the bit rate recognition by means of thecircuit block 4 as accurately and securely as possible, a specialevaluation of several fields of the header that precedes every datablock is executed as invented. This is further explained as follows withreference to FIG. 2, which gives a schematic representation of thestructure of such a header with three data packets.

As FIG. 2 shows, the whole header as shown in FIG. 2 has a maximum of 34bits. The header is subdivided into three fields, namely a sync breakfield, a sync field and an ID field. Here the sync break field, the syncfield and the ID field have one start bit each; the sync field and theID field have one stop bit each and the sync break field has at leastone stop bit.

With the solutions known according to the state of the art, only thesync field is used for bit rate recognition in slave bus subscribers.Its first 8 bits represent bits having known positions and known values,so that the time intervals to the bit edges of these bits can beevaluated and used for bit rate recognition. The main disadvantage ofthis is that only 8 bits are used for bit rate recognition and thus theaccuracy of the bit rate recognition is relatively low. Furthermore, thelow number of evaluated bits causes relatively high sensitivity tointerference and jitter and to superpositioning of this data businterference.

The bit rate recognition 4 in the slave bus subscriber as invented istherefore designed, as shown in FIG. 1, so that that also the bitshaving known positions and known values of the other fields, viz. of thesync break field and ID field are evaluated.

The representation in FIG. 2 shows where consecutive bits haverespectively changing bit values. The sync break field has 14 bits,which are set to 0 in this example, with the exception of the stop bit.The ID field has 10 bits, whose contents may perhaps not be known, withthe exception of the start and stop bits.

The first two fields each have a start and stop bit at the beginning andat the end respectively, the start bit at the beginning of the fieldhaving the 0 value and the stop bit at the end of the field having the 1value. This similarly holds for the ID field. The sync break field mayalso have more than one stop bit, which can be communicated to the slave1 possibly by using a corresponding signal or through programming.

Thus, these bit positions or edges of these bits as well as the usefulbits of these fields can be used for the bit rate recognition. In thesimplest case, up to 24 bits of the sync break field and of the syncfield can be used for the bit rate recognition. Also 7 useful bits ofthe ID field as well as its start bit may be used for bit raterecognition, so that the bit rate recognition can reckon with atransmission time of 24 bits or 34 bits and thus the operation can beclearly more accurate than if it were based only on a transmission timeof 8 bits, which is the case for solutions in accordance with the stateof the art.

The slave bus subscriber as invented has been described in the exampleof embodiment with reference to a bus subscriber for the LIN data bus;the invention, however, is not restricted to a slave bus subscriber forthis type of data bus, but can rather be used in many cases for serialdata buses having a header of a known structure.

1. A slave bus subscriber for a serial data bus with a master bussubscriber, wherein the slave bus subscriber recognizes with the help ofthe header of the data packet the bit rate of a data packet whose headerhas a sync break field, a sync field and an ID field and is receivedover the data bus, so that the periods between edges of bits havingknown bit intervals of at least the sync field and the sync break fieldare evaluated and the bit rate is determined from the evaluated periods,wherein the slave bus subscriber evaluates at least periods betweenfalling edges of a first bit of the sync break field, of a plurality ofbits of the sync field and at least a first bit of the ID field.
 2. Theslave bus subscriber of claim 1, wherein the sync break field, the syncfield and the ID field have one start bit each, the sync field and theID field have one stop bit each, and the sync break field has at leastone stop bit.
 3. The slave bus subscriber of claim 2, wherein as long asa number of stop bits of the sync break field is not a priori known tothe slave bus subscriber, the number of stop bits can be indicated tothe slave bus subscriber or programmed in it.
 4. The slave bussubscriber of claim 2, wherein a number of the stop bits of the syncbreak field greater than one bit is taken into account when the periodsare determined.
 5. The slave bus subscriber of claim 1, wherein theslave bus subscriber also evaluates the period positions of the fallingedges of known bits of the ID field.
 6. The slave bus subscriber ofclaim 1, wherein the serial data bus is a LIN data bus.
 7. A slave bussubscriber for a serial data bus with a master bus subscriber,comprising: a transceiver that receives a data packet having a header, async break field, a sync field, and an ID field; and a bit rateidentifier that identifies a bit rate of the data packet based on thesync break field, the sync field and the ID field including evaluationof periods between edges of bits of the header having known bitintervals of at least the sync field and the sync break field anddetermining the bit rate from the evaluated periods, and includingevaluation of period positions of falling edges of known bits of the IDfield.
 8. The slave bus subscriber according to claim 7, characterizedin that the slave bus subscriber evaluates at least the periods betweenthe falling edges of the first bit of the sync break field, of five bitsof the sync field and at least of the first bit of the ID field.
 9. Theslave bus subscriber of claim 7, wherein the bit rate identifieridentifies bit rates of data packets wherein the sync break field, thesync field and the ID field have one start bit each, the sync field andthe ID field have one stop bit each, and the sync break field has atleast one stop bit.
 10. The slave bus subscriber of claim 9, wherein thebit rate identifier identifies bit rates of data packets as long as anumber of stop bits of the sync break field is not a priori known to theslave bus subscriber, the number of stop bits can be indicated to theslave bus subscriber or programmed in it.
 11. The slave bus subscriberof claim 9 wherein the bit rate identifier identifies bit rates of datapackets wherein a number of the stop bits of the sync break fieldgreater than one bit is taken into account when the periods aredetermined.
 12. The slave bus subscriber of claim 8, wherein the bitrate identifier identifies bit rates of data packets when the serialdata bus is a LIN data bus.